1. Field of the Invention
The present invention relates to a printer capable of printing images on both sides of a sheet in a single step.
2. Description of the Background Art
A digital, thermal printer using a stencil is extensively used as a simple, convenient printer and includes a thermal head provided with an array of minute heat-generating elements. While a thermosensitive stencil is being conveyed in contact with the thermal head, a current is selectively fed to the heat-generating elements in the form of pulses in accordance with image data, thereby perforating the stencil with heat. After the perforated stencil or master has been wrapped around a porous, cylindrical print drum, a press roller or similar pressing means is pressed against the print drum via a sheet. As a result, ink is transferred from the print drum to the sheet via the perforations of the master, printing an image on the sheet.
Today, duplex printing that prints images on both sides of a sheet is spreading because it reduces the consumption of sheets, the space for storing of documents and so forth. It has been customary to execute duplex printing by passing a sheet fed from a sheet feeding section through a printing section to thereby print an image on one side of the sheet, reversing the sheet, and again passing the sheet through the printing section to thereby print another image on the other side of the sheet. However, it is troublesome to again set the sheet driven out at the sheet feeding section or to arrange consecutive sheets each carrying an image on one side thereof by hand.
Further, when an image is printed on the reverse side of a sheet just after the printing of an image on the front side, the image on the front side, which is still wet, is blurred or otherwise disturbed when various rollers including the press roller contact it. For this reason, the printing of an image on the reverse side is, in many cases, effected on the elapse of several hours since the printing of an image on the front side. This is particularly true with an image including a solid portion having a substantial area. In this manner, the conventional duplex printing scheme needs a long period of time for drying an image printed on one side of a sheet. In addition, duplex printing time is two times as long as simplex printing time because a single sheet must be conveyed via the printing section two times.
In light of the above, Japanese Patent Laid-Open Publication Nos. 6-71996 and 6-135111 each disclose a stencil printer including a first and a second print drum and moving means for selectively causing the two print drums into or out of contact with each other. The stencil printer produces a duplex print in a single step by causing the two print drums to contact with each other with the moving means.
Also, Japanese Patent Laid-Open Publication Nos. 8-90893 and 8-142477 each propose a stencil printer including a first print drum, first pressing means facing the first print drum via a sheet path and movable into and out of contact with the print drum, a second print drum positioned downstream of the first drum in a direction of sheet conveyance and facing the first drum via a sheet path, and second pressing means facing the second print drum via a sheet path and movable into and out of contact with the second drum. To produce a duplex print in a single step, the first drum and first pressing means and the second drum and second pressing means are sequentially caused to contact each other in this order.
Further, Japanese Patent Laid-Open Publication No. 8-332768 teaches a stencil printing method and a stencil printer for practicing the same. The stencil printing method taught in this document produces a duplex print in a single step, which consists of a first and a second step, by use of a master formed with a first and a second image side by side in the direction of rotation of a print drum. In the first step, a press roller is rotated in direct contact with the print drum in synchronism with either one of the first and second images, so that an image corresponding to the first or the second image is transferred from the print drum to the press roller. In the second step, the press roller is rotated in contact with the print drum via a sheet with the other of the first and second images being matched in position to the image present on the press roller. As a result, the image on the press roller is transferred to a first side of the sheet while an image corresponding to the second image is transferred from the print drum to the other side of the sheet.
Laid-Open Publication Nos. 6-71996 and 6-135111 stated above have the following problems left unsolved. The two print drums positioned one above the other are configured to be selectively brought into or out of contact with each other and are done so even in a simplex print mode. In the simplex mode, a perforated, or cut, master and a non-perforated master must be respectively wrapped around the two print drums, resulting in the wasteful consumption of a stencil. Further, when dampers mounted on the two print drums face each other, the print drums must be released from each other. This brings about a problem that when printing speed is high, the area over which the print drums contact each other and therefore an image area decreases. If the outside diameter of each print drum is increased to guarantee a sufficient image area, then not only the size reduction of the printer is obstructed, but also loud noise is produced when the print drums are brought into contact.
Laid-Open Publication Nos. 8-90893 and 8-142477 also have the problem that a non-perforation master must be wrapped around one of the two print drums in a simplex print mode, resulting in the wasteful consumption of a stencil. Another problem is that because the two print drums are serially arranged, the printer is almost two times as large in size as a stencil printer for simplex printing. This is undesirable from the space saving standpoint.
Further, the problem with Laid-Open Publication No. 8-332768 is that image density differs from the front side to the rear side of a sheet because one of the first and second images is directly transferred from the print drum to a sheet while the other image is transferred to the sheet by way of the press roller.
It is an object of the present invention to provide a printer capable of producing a simplex print in a simplex print mode without wasting a stencil or producing an attractive duplex print in a duplex print mode while occupying a minimum of space.
A printer with a duplex printing capability, of the present invention includes a printing section including a print drum around which a master formed with a first and a second image side by side is to be wrapped and a press roller selectively movable into or out of contact with the print drum. A sheet feeding section feeds sheets toward the printing section one by one. A sheet discharging section discharges a printed sheet coming out of the printing section to the outside of the printer. An auxiliary tray temporarily stops the printed sheet carrying an image on its front side thereof. A refeeding device again feeds the printed sheet temporarily stopped by the auxiliary tray toward the printing section. A path selector selectively steers the sheet coming out of the printing section toward the auxiliary tray or the sheet discharging section. In a duplex print mode, the printing section prints either one of the first and second images on the front side of the first sheet fed from the sheet feeding section. The path selector steers the first sheet coming out of the printing section toward the auxiliary tray. The printing section then prints either one of the first and second images on the front side of the second sheet fed from the sheet feeding section while the refeeding device again feeds the first sheet to the printing section to thereby form either one of the first and second images on the reverse side of the first sheet. The path selector steers the first sheet toward the sheet discharging section and steers the second sheet toward the auxiliary tray.